1. Field of the Invention
The invention relates to a method of detecting breakage of a tool and, more particularly, to such a method as to quickly detect the breakage.
2. Description of the Related Art
Conventionally known is a machine too1 adapted to rotate a tool on an axis of the tool to machine a workpiece with the tool and further adapted to automatically change the tool with another tool. As described in Japanese Patent Publication No. 52-34075, for example, this type of machine tool is constructed so as to include (a) a rotary spindle having one end for holding a tool, (b) a tool magazine for accommodating a plurality of kinds of tools, and (c) a moving device for moving the spindle along a predetermined moving path to a tool changing position where the tool held by the spindle is changed with one of the tools accommodated in the tool magazine.
In such a machine tool, breakage of the tool sometimes occurs. It is therefore necessary to detect the breakage and exchange the broken tool with another tool. The machine tool described in the above citation is provided with a breakage detecting device for detecting the breakage of the tool after the end of machining.
In this machine tool having the breakage detecting device, the spindle holding the tool is adapted to be rotated on its horizontally extending rotation axis and moved in a vertical direction. The workpiece is fixedly positioned on a table movable in a first direction parallel to the rotation axis of the spindle and a second direction perpendicular to the first direction in the same horizontal plane. Accordingly, the workpiece is desirably machined by the movements of the spindle and the table. The breakage detecting device is provided on the table, at an end portion thereof, on the spindle side. The breakage detecting device includes a sensor arm mounted on the table so as to be swingable on an axis parallel to the rotation axis of the spindle, a sensing element provided at one end of the sensor arm so as to be movable in a direction parallel to the swing axis of the sensor arm, a spring for biasing the sensing element toward the tool, and a limit switch adapted to be operated by retraction of the sensing element.
During machining of the workpiece, the sensor arm is kept in a retracted position where it extends horizontally, while upon detection of the breakage, the sensor arm is swung to a detecting position where it extends vertically. Further, upon detection of the breakage, the spindle is raised to a position where the tool held by the spindle is aligned with the sensing element of the sensor arm swung to the detecting position. In this condition, the sensing element is advanced toward the tool by -moving the table. The amount of movement of the table is set according to the length of the tool. If the tool is not broken, the sensing element comes into contact with the tool, and is then retracted to operate the limit switch, thereby judging a breakage has not occurred. On the other hand, if the tool is broken, the sensing element is not retracted, and the limit switch is accordingly not operated, thereby judging a breakage has occurred.
In the above breakage detecting device, however, it is necessary, for detecting the breakage, to swing the sensor arm, move the tool to the position opposed to the sensing element and, then, move the sensing element in the direction parallel to the rotation axis of the tool. Thus, much time is required for the breakage detection.